Blood, emulsions, pharmaceuticals, beverages, and other items are often chilled in order that they may be preserved for later use. If exposed to too low a temperature, however, the characteristics of these items may change so that they are undesirable for later use. In fact, many of these items may be rendered completely useless if exposed to too low a temperature, such as the freezing point of water. It is, therefore, desirable to provide an indicating device that will accurately show whether or not the item is exposed to a temperature below a critical value. It is also typically desirable for the device to provide its indication relatively rapidly and that the indication be irreversible so that the user will be alerted to a previously attained critical temperature, even if that condition is not maintained.
Critical temperature indicating devices are known that utilize the expansion characteristic of water to break a frangible ampule. That is, once the indicating device is exposed to a temperature below the freezing point of water, the volume increases as the water in the ampule changes to ice, thereby causing the ampule to break. After the ice is formed and the surrounding temperature returned to a point about the melting point of the ice, the resultant water is absorbed by a pad, thus providing an indication that the device had gone through a freeze stage and back through a thaw stage. Such indicating devices are not always accurate due to the super-cooling effect of water. Under some conditions, water may be cooled to as low as -16.degree. C. without freezing. This problem has been partially overcome by the addition of nucleating agents to the water to enhance ice formation.
Water-based indicating devices are also known that utilize a microporous layer, water, and a second liquid that can wet out the microporous layer upon solidification of the water. See, U.S. Pat. No. 4,846,095 (Emslander). The micropores of the microporous layer cause scattering of transmitted light, causing the microporous layer to appear opaque. When the voids are filled with a liquid having substantially the same index of refraction as the material of the microporous layer, the filled voids do not significantly scatter transmitted light, thus rendering the microporous layer transmissive to visible light. The opacity of the microporous layer masks a visible indicator, which can be seen after the indicator is activated (i.e., after the second liquid enters the voids of the microporous layer). Emslander also discloses that a seeding agent or a salt can be added to the aqueous mixture to slightly modify the indicating temperature. Emslander further discloses that two nonaqueous compounds (ethanolamine and n-butanol), one of which solidifies and one of which wets out the microporous layer, can be used in the indicating devices containing a microporous layer.
Other temperature indicating devices are known that include organic compounds in a capillary tube or bulb attached to a capillary tube. See, for example, U.S. Pat. Nos. 4,457,252 and 4,457,253. These indicating devices utilize the volume reduction characteristics of organic compounds as they undergo a change from the liquid state to the solid state. Two organic liquids are separated by a porous plug and/or a separating liquid that is immiscible with the two organic liquids. One of the organic liquids is colored and has a freezing point that is higher than that of the other (colorless) organic liquid. At the critical temperature, the colorless organic liquid solidifies and the colored liquid is drawn into that portion of the capillary tube or bulb containing the colorless, solidified organic liquid, thereby providing a visual indication that the first compound solidified.
Even with these nonaqueous indicating devices, there is a need for additional indicating devices, particularly those that are responsive relatively rapidly and reproducibly within a relatively narrow temperature range. Furthermore, there is a need for additional indicating devices that are capable of being readily modified to a wide range of critical temperatures.